In present data processing systems a logic tree is included to sequence the turn-on or turn-off of the power supplies when the system is started up or shut down. The logic tree is a series of sequential logic circuits arranged to turn on one voltage supply, check that this voltage supply exceeds a predetermined threshold and then turn on the next voltage supply until the whole system is powered. In addition, a meter is included for the service engineer who may switch the meter to various points throughout the system to check that voltage levels are within tolerance.
A description of such a known power control system is given in IBM Maintenance Library Manuals: IBM 3125 Processing Unit, Power Supplies, Order No. SY33-1060-3; IBM 3115 Processing Unit, Power Supplies Order No. SY33-1075-2. (IBM is a Registered Trade Mark of International Business Machines Corporation).
Modern data processing systems have become extremely complex and it would be desirable to implement a power control system having complex control functions in excess of the basic functions mentioned above. Such a sophisticated power supply control system could be implemented by increasing the complexity of the conventional control logic tree. Apart from the cost of this approach, it involves the disadvantage that a logic tree is difficult to change so the system is inflexible. Thus, any change in the power supplies requires a change in wiring of the logic tree or a change in the logic of the tree which may make the implementation of changes expensive and time consuming.